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Vegetables and Fruits and the Risk of Breast CancerFact Sheet #18, June 2005

Recent studies have challenged prior thinking about the relationship between vegetable and fruit consumption and breast cancer risk. The addition of the results of recent cohort studies to the body of evidence on this
subject has brought into question the existence of a strong association between vegetable and fruit consumption and decreased breast cancer risk. It now looks as if fruit consumption is not associated with breast cancer risk. The role of vegetable consumption is unclear; it may or may not be associated with a decrease in breast cancer risk. It appears that if an association of vegetable consumption with breast cancer risk exists, it is small. Comparisons of women eating a Western-style vegetarian diet compared to those eating conventional Western diets have also not detected any association with breast cancer risk. However, high consumption of vegetables after breast cancer diagnosis shows promise for decreasing the risk of death for survivors, but more study is needed before this effect is established.

Numerous health benefits are associated with eating vegetables and fruits. A diet high in vegetables and fruits is likely to be linked to a decrease in the risk of cancer of the mouth, esophagus, stomach, lung, colon, rectum
and possibly other cancers. Eating large amounts of vegetables and fruits has been clearly linked to a decrease in the risk of cardiovascular disease. Eating vegetables and fruits may also help improve bone health, diabetes
control, and the maintenance of a healthy body weight.

Does the total amount of all vegetables and fruits consumed affect breast cancer risk?

Recent cohort epidemiological studies examining the association between vegetables and fruits consumption
and breast cancer risk present two new findings. First, breast cancer risk appears to be unrelated to the total
amount of fruit consumed. Second, the relationship of total vegetable consumption to breast cancer risk is
uncertain. If a relationship does exist, it is relatively small, on the order of a 10 percent to 20 percent decrease in risk. It is important to recognize that this conclusion is based on studies of vegetables and fruits as entire food groups and that a number of scientists are critical of this conclusion. Studies that group all types of vegetables or fruits and treat the groups as one would not show the effects of specific vegetables and fruits (see “Are there certain vegetables and fruits that may affect breast cancer risk?” below). In addition, there are clear health benefits associated with vegetable and fruit consumption (see “Is eating vegetables and fruits associated with other health benefits?” and “Is eating vegetables and fruits
associated with changes in the risk of other types of cancer?” below).

Previously vegetables and fruits were thought to be linked to a decrease in breast cancer risk. Why has this idea changed?

Thought about the relationship between breast cancer
risk and vegetables and fruits changed largely because of
the results of recent cohort studies. Cohort studies have
almost uniformly found no association between total
consumption of fruits or vegetables and breast cancer
risk. Previous thinking had been based largely on casecontrol
studies. Although the results of the case-control
studies varied, the majority had reported a decrease in
breast cancer risk. Case-control and cohort studies have
different designs. Case-control studies compare the past
diet as remembered by women with breast cancer with
the past diets of similar women without breast cancer.
Cohort studies assess the diets of healthy women and
then follow them over time for the appearance of breast
cancer and other diseases. The data obtained using this
approach is less likely to be biased and these studies are
considered more reliable. The results of the cohort
studies have led many researchers to reevaluate the link
between vegetables and fruits and decreased breast
cancer risk.

Are there certain vegetables and fruits that may affect breast cancer risk?

It is possible that specific vegetables and fruits or
combinations of vegetables and fruits may decrease
breast cancer risk. Vegetables and fruits contain a
number of chemicals, such as the dithiolthiones and
glucosinolates from cruciferous vegetables, which have
been demonstrated to inhibit the growth of cancer cells
and cancer formation in animals. These chemicals have
also been linked to decreased risk of other cancers. (See
“What do vegetables and fruits contain that could influence
the risk of breast cancer?” below). However, most
existing studies of large groups of women have not
examined vegetables and fruits individually. The results
of those studies that have looked at individual foods are
conflicting. One notable study, a pooled analysis, which
gathered, combined, and reanalyzed the data from
eight different cohort studies (total of 7,377 breast
cancer cases) did not find a change in breast cancer risk
when vegetables or fruits were examined as botanical
families. They did observe decreases in risk linked to
broccoli, Brussels sprouts, and spinach. But none of
these reductions in risk were statistically significant,
which decreases confidence in the results. Some casecontrol
studies have shown promise for carrots and
spinach as well as members of the broccoli (cruciferous)
family. Some of these studies also found a significant
dose response (eating more vegetables lead to greater
reductions in risk). Both green and yellow vegetables
have also shown promise in some studies. In addition,
there is limited evidence that raw vegetables may be
more protective against the development of breast and
other cancers than cooked vegetables. This may be
because some of the potentially protective chemicals
found in vegetables are damaged by heat.

Citrus fruits have also been studied individually. The
results of these studies have been variable. Any connection
between citrus consumption and a change in breast
cancer risk is uncertain.

What do vegetables and fruits contain that could influence the risk of breast cancer?

Researchers have identified and isolated many natural
chemicals in vegetables and fruits (phytochemicals) that
affect cancer risk. Some of these chemicals are shown in
Table 1 below. Although these chemicals are available as
supplements, it is important to recognize that supplements have, in most cases, not shown the benefits of the
whole foods containing these chemicals.

There are many other natural chemicals in vegetables
and fruits that researchers are currently studying.
Information on the cancer preventing properties of these
chemicals is preliminary and their concentration in
various vegetables and fruits is still being determined.
These natural chemicals include isothiocyanates and
thiocyanates (in Brussels sprouts), flavonoids (in berries),
coumarins (in citrus fruits), phenols (in almost all vegetables
and fruits), protease inhibitors (in legumes),
plant sterols (in vegetables), isoflavones, saponins, and
inositol hexaphosphate (in soybeans), allium compounds
(in garlic), limonene (in citrus fruit oils), and
resveratrol (in grapes).

How might vegetables and fruits influence the risk of breast cancer?

It is biologically feasible that vegetable and fruit
consumption could affect breast cancer risk. There are
several ways that the natural chemicals found in
vegetables and fruits might help reduce the risk of breast
cancer and other cancers. Some of the mechanisms are
listed below.

Stimulate cell differentiation and stop cell proliferation.
Cell differentiation is the process by which a cell in the
body becomes functionally mature. Differentiated cells
have a low proliferation rate. Cancer arises largely
from proliferating cells which are not differentiated.
Compounds in vegetables and fruits, such as
carotenoid-derived vitamin A, can encourage cells to
differentiate and potentially protect against cancer
formation.

Act as antioxidants.
Cells are exposed to oxidants from oxygen, some
products of metabolism and from oxidant-producing
toxins. Oxidants can damage various parts of cells
including their DNA. Such damage can potentially
lead to cancer formation. Vegetables and fruits
contain many antioxidants, such as vitamin C and
carotenoids. These chemicals can neutralize oxidants.

Increase activity of protective detoxifying enzymes.
Cells in the body are exposed to various toxins including
cancer-causing compounds. These toxins can be
deactivated and eliminated from the body by protective
enzyme systems. Some chemicals in vegetables
and fruits, such as dithiolthiones in broccoli, have
been shown to increase the activity of the protective
enzyme systems in the body.

Enhance immune function.
The consumption of vegetables and fruits may
strengthen the immune system, which is the body’s
defense against various diseases including cancer.

Alter estrogen levels.
Estrogen is a hormone that is necessary for childbearing,
but higher lifetime exposure to estrogen’s actions
is associated with higher breast cancer risk. Estrogen
is normally metabolized to forms that have different
strengths of action. Vegetables and fruits have compounds,
such as glucosinolates in broccoli, that
increase the metabolism of estrogen to weaker forms.
This effect may change the lifetime exposure to estrogen’s
actions and decrease breast cancer risk.

Compete with naturally produced estrogen.
Phytoestrogens, or plant estrogens, may be weaker
than the estrogens which occur naturally in the body.
The weaker phytoestrogens may compete with naturally
occurring estrogens and block their effects (some
of which are linked to breast cancer risk). However,
some studies have raised concern that phytoestrogens
may not be so weak, and they may increase
rather than block estrogen activity (see BCERF Fact
Sheet #1, Phytoestrogens and the Risk of Breast
Cancer).

Are vegetarian diets associated with changes in breast cancer risk?

No clear link between vegetarian diets and a change in
breast cancer risk has been found. Most studies have
examined Western-style vegetarianism in women in the
U.S., United Kingdom, and Germany. Five cohort studies
have evaluated the risk of death from breast cancer in
vegetarians compared to non-vegetarians. The data
from these studies has been pooled and reanalyzed. This
analysis found no association between death from breast
cancer and eating a vegetarian diet. Although these
results are weakened because the researchers did not
account for known breast cancer risk factors in their
analysis, all but one of the included studies came to the
same conclusion. Further, the one conflicting study
reported a statistically significant increase in breast
cancer risk.

A problem with this area of research is that western
style vegetarian diets are quite varied. Few studies have
examined more defined vegetarian diets. One such
study examined non-western vegetarian women living
in England who had migrated there from India or east
Africa. Most of these women were of the Hindu religion
and did not eat any meat or eggs but did consume dairy
products. In this study, women who were lifelong
vegetarians or vegetarians since adulthood had a
decreased risk of breast cancer compared to lifelong
meat eaters. However, these results were weakened by a
lack of statistical significance. Nonetheless, there were
significant decreases in risk for women with high
consumption of either vegetable dishes or dishes with
peas, beans or lentils. Both these results were also
supported by the demonstration of a significant doseresponse
effect for the relationship of the decrease in risk
and the amount of the foods typically eaten.

Other studies of vegetarians have evaluated their
levels of the hormone estradiol and one of its protein
carriers in the blood. Estradiol is the most potent
estrogen and its levels in the body have been associated
with breast cancer risk. The steroid hormone binding
globulin, one of the estrogen protein carriers, can
strongly bind to estradiol and decrease its biological
availability and action. Several studies have examined
the levels of estradiol and the steroid hormone binding
globulin in premenopausal vegetarians and nonvegetarians.
None of these studies have found a
difference that was statistically significant between the
vegetarians and non-vegetarians. The largest and best
conducted of these studies reported that body mass
index differences may have an important role in any
differences found between these groups. Vegetarians
have been shown to have less body fat and women’s
body fat can affect the levels of estradiol and the binding
protein.

Does the consumption of vegetables and fruits have different effects among premenopausal versus postmenopausal women?

The relationship between vegetable and fruit consumption
and breast cancer risk does not appear to be affected
by women’s menopausal stage. The causes of preand
postmenopausal breast cancer, although not well
understood, may be different. Thus, it is possible that
their risk factors could be different. Several studies have
reported that premenopausal women who ate a lot of
vegetables had a larger decrease in breast cancer risk
than postmenopausal women who ate a lot of vegetables.
But most studies, including a pooled analysis of
cohort studies, have not detected any risk difference
between pre- and postmenopausal women.

Could some women benefit more than others from eating vegetables and fruits?

There are suggestions but no clear demonstration that
some women may benefit more than others from eating
vegetables and fruits. For example, one cohort study
conducted as part of the Nurses Health Study examined
the breast cancer risk of premenopausal women with a
family history of breast cancer. This study found that
women with a family history of breast cancer who ate
the most vegetables and fruits had a relative risk of
breast cancer which was less than one third (29 percent)
that of similar women who ate the least vegetables and
fruits.

Other supportive studies have examined the effects
of diet in women with differences in genes which are
thought to protect against cancer. Three studies support
the idea that one form of the gene for the enzyme
manganese superoxide dismutase is linked to increased
breast cancer risk. The enzyme product of this gene can
inactivate toxic free radicals and is thought to play a role
in the defense against cancer. One of these studies also
examined vegetable and fruit consumption in women
who had and did not have the high risk form of the
gene. They found that women who ate large amounts
vegetables and fruits and had the high risk gene still had
elevated risk of breast cancer but their risk was
substantially less than women with the high risk gene
who ate small amounts of vegetables and fruits. Similar
suggestions for other genes and types of cancer exist in
this area of research which is rapidly expanding.

Is eating vegetables and fruits helpful for women who already have breast cancer?

Studies of the effect of diet after diagnosis on the survival
of women with breast cancer show promise for
vegetables but not fruits. While the increase in survival
has only been evaluated in two studies and is not established,
it was of reasonable size. One of these studies
examined the Nurses Health Study cohort and reported
that in a group of 1,237 women whose breast cancer
had not metastasized to the lymph nodes, the highest
levels of vegetable consumption were significantly associated
with as much as a 47 percent decrease in the risk
of death. A smaller study of 377 women reported an
even more substantial but not statistically significant
decrease in the risk of death. Some studies which evaluated
diet before diagnosis have also reported a beneficial
effect. More study is needed to confirm this effect. For
more information please see BCERF Fact Sheet #44 Diet
and Lifestyle and Survival from Breast Cancer.

Is eating vegetables and fruits associated with changes in the risk of other types of cancer?

The International Agency for Research on Cancer
recently evaluated the evidence for a link between a diet
rich in fruit and vegetables and decreased cancer risk.
They concluded that the evidence from human studies,
animal studies, and mechanistic studies, indicates: “that
higher intake of fruit probably lowers the risk of cancers
of the esophagus, stomach and lung, while a higher
intake of vegetables probably lowers the risk of cancers
of the esophagus and colon-rectum. Likewise, a higher
intake of fruit possibly reduces the risk of cancers of the
mouth, pharynx, colon-rectum, larynx, kidney and
urinary bladder. An increase in consumption of
vegetables possibly reduces the risk of cancers of the
mouth, pharynx, stomach, larynx, lung, ovary and
kidney.”

Recent studies have changed the prevailing view of
the association of a diet high in vegetables and fruits
with the risk of cancer, in general. The idea that the relationship
is strong and that it broadly affects many cancers
has been brought into question. The prevailing
opinion is that a decrease in risk exists for the cancers
listed above and that there is potential for diets high in
vegetables and fruits to have modest benefits for cancer
prevention in general.

Is eating vegetables and fruits associated with other health benefits?

A diet high in vegetables and fruits has been clearly
linked to a decrease in the risk of the major cause of
death in this country, cardiovascular disease. Both types
of cardiovascular disease, coronary heart disease and
stroke, are affected. Several studies of the largest cohorts
have found a statistically significant 20 percent decrease
in the risk of heart disease and a 30 percent decrease in
the risk of stroke for a group that included men and
women. There also appeared to be a trend or doseresponse
for this effect, that is, eating more of these
foods was progressively linked to a greater decrease in
risk. The lowest risk of coronary heart disease and stroke
was reported for the group that ate the most servings of
vegetables a day. Intermediate changes in the risk of
both of these diseases were linked to eating smaller
amounts of vegetables and fruit.

A positive effect of vegetable and fruit consumption
on bone health has also been demonstrated in recent
years. The size of this effect remains to be seen.
Although vegetables and fruits are not directly involved
in diabetes and obesity, a diet high in vegetables and
fruits is frequently recommended as a prudent preventive
measure for these diseases.

What can women do now?

Although the health benefits of vegetables and fruits,
especially against cancer, are not as great as once
thought, they still remain substantial. It makes good
sense to increase the number of servings and variety of
vegetables and fruits eaten daily. In addition, eating
plenty of vegetables and fruits helps in maintaining a
healthy weight.

There are many ways to easily and conveniently add
more vegetables and fruits to our diets.
Easy examples would include the following:

Keeping prepared vegetables in the refrigerator for snacks

Substituting spinach or another dark green leafy vegetable for iceberg lettuce in salads or sandwiches

Eating a sweet potato instead of a white potato

Eating fruit as a snack

Drinking fruit or vegetable juice instead of soda

Think of lentil and bean soups when your menu calls for a hearty meal.